Early studies examining the pathways of class I and class II restricted antigen presentation suggested a clear demarcation between these two pathways in terms of their antigenic sources. Class II-restricted antigens were derived from exogenous proteins processed in the cytoplasm. The exclusion of exogenous antigens from the class I pathway made sense in that this pathway was considered to be for the sensitizing of viral-infected cells for lysis by CD8 cytotoxic T lymphocytes (CTL). Thus, only cells containing the viral proteins (and therefore the virus) could become targets for CTL destruction. Evidence for this model of class I-restricted presentation was obtained by examining the requirements for sensitizing target cells for killing by effector CTL. When the priming of naive CD8 T-cells was examined, quite a different conclusion emerged. Exogenous proteins were reported to prime various CD8 T-cell responses, including those to minor HAs, viral and tumor antigens - this was referred to as cross-priming, and the pathway as the cross-presentation pathway. These studies implied that under certain conditions exogenous antigens could enter the class I pathway of antigen presenting cells (APC), be processed into the class I pathway, and prime CD8 T-cells. Recently, using transgenic mice expressing neo-self antigens in tissues such as the pancreas, the investigators have begun to characterize this pathway. Self antigens were shown to be constitutively presented in a class I-restricted manner by a bone marrow-derived APC that resides in the lymph nodes. These data suggest that antigens expressed in tissues are captured by APC, transported to the draining lymph nodes, and presented to CD8 T-cells. They have shown that this provides a pathway for induction of peripheral tolerance, as well as immunity, and that several factors affect which of these outcomes emerges, including the availability of cognate CD4 T-cell "help". The hypothesis is that the cross-presentation pathway plays an important role in the surveillance of tissues for viral and tumor antigens, and in the maintenance of self tolerance to peripheral antigens. This application plans to utilize the transgenic models to characterize both peripheral tolerance and immunity induced by the cross-presentation pathway. Specifically, the application will examine: (i) the nature of antigens that can be presented by this pathway; (ii) the mechanism by which this pathway induces tolerance to self antigens; and (iii) how CD4 T-cells provide help for generation of CTL immunity.